In metazoans, many cellular proteins are regulated by posttranslational modification of serine and threonine residues with N-acetylglucosamine to create an O-GlcNAc appendage. Acylation may compete for the same residues as phosphorylation, but in contrast to the hundreds of kinases and phosphatases that regulate phosphorylation status, there is but a single O-GlcNAc transferase (OGT) and a single O-GlcNAc hydrolase. Because no acylation sequence motif has been identified as yet, how OGT recognizes its protein substrates remains unclear.

Clarke et al. report the structure of OGT both in the apo state and in complex with a phosphonate analog of the sugar donor UDP-GlcNAc. The N-terminal domain comprises repeats of the tetratricopeptide (TPR) protein interaction module and the catalytic C-terminal domain has two subdomains that together form the metal-independent glycosyltransferase fold GT-B. The UDP-GlcNAc analog is pinched between the subdomains at the bottom of a conserved groove that is suitably shaped to bind substrate peptides; the TPRs interact with the catalytic domain so that the putative substrate binding groove extends into the TPR domain. The authors present mutagenesis data consistent with the proposal that peptides bind in the active-site groove, with proteins binding to the extended surface formed by the TPRs. A phosphoinositide binding site is located on the surface of the catalytic domain and may recruit OGT to membrane-bound targets. — VV